The present invention is related to active electronic filters, and more particularly to a gm-C filter with an adjustable cutoff frequency.
Active filters, such as a typical (low pass) gm-C filter 10 illustrated in FIG. 1, may include circuitry at output node N for adjusting the cutoff frequency of the filter. In a gm-C filter the cutoff frequency may be adjusted by varying the value of Gm in gm stages 12 of filter 10. However, this may distort the signal and a preferable cutoff frequency adjustment technique is to provide an array of capacitors 14, such as illustrated in FIG. 2, to replace the single capacitor C in FIG. 1. The cutoff frequency may be adjusted by selectively connecting one or more of capacitors 14 in the array, such as by closing switches 16. The switches may be MOS switches and the operation of the switches may be programmed for use in devices such as cordless telephones and wireless speakers.
Even this technique is not without problems. When one of capacitors 14 is selected, the stray capacitance (represented by capacitors 18) of deselected capacitors 14 will unacceptably increase the total capacitance of the capacitor array. That is, C(total)=C(selected capacitors)+stray C(deselected capacitors). The effect is especially noticeable when a small capacitor 14 is selected and stray capacitance from a much larger capacitor is present.
To avoid this problem, it is known to move switches 16 to the connection between node N and capacitors 14, such as illustrated by switches 16' shown in dashed lines in FIG. 2. This removes the problem of the stray capacitance, but causes a new problem. In devices in which the supply voltage is small, there may be insufficient gate drive to completely turn on switches 16', and if the supply voltage is increased there may be resistance added that would cause the filter response to vary from ideal. For example, if switches 16' are MOS devices (located between node N and capacitors 14), the top plate of selected capacitor 14 will be forced to a DC level approximately at mid supply. The resulting bias on the MOS devices would generate an unpredictable (and thus unacceptable) amount of series resistance to the circuit. Even if the supply voltage is increased to assure that the MOS switches are activated, the finite amount of added resistance may cause variations in filter response.